This invention relates to electrophotography and in particular relates to fusing, glazing and transparentizing of colored electrophotographic toner deposits contained on receiving member surfaces such as paper.
It is known to produce color prints by electrophotographic processes, such prints being commonly used as lithographic or gravure pre-press proofs, containing in general four colors, such as yellow, magenta, cyan and black. Such pre-press proofing processes are disclosed for instance in U.S. Pat. Nos. 3,337,340; 3,419,411; and 3,862,848.
It is customary to produce such electrophotographic pre-press proofs by charging a photoconductive recording member followed by exposure through a separation transparency corresponding to one color, followed by toning of the exposed photoconductor with a liquid dispersed toner of the appropriate color, followed by in-register transfer of the color toned image deposit to a receiving member surface, such as paper. These process steps are then repeated with separation transparencies of the other three or more colors and appropriate color toners to produce a multicolor print as required.
After all of the required color toner deposits have been transferred to the receiving member paper sheet, it is coated with a clear resin layer to transparentize the color toner deposits and fuse them to the paper sheet. Such coating may be carried out by spraying, curtain, roller or dip coating and the like.
The primary purpose of pre-press proofs is to assess color balance and strength which can be expected from the final press run and accordingly to correct the separation transparencies before the printing plates are made therefrom. In many instances, it is also required to produce socalled customer proofs for approval of subject, composition and general appearance of the print prior to press run. Thus, it is essential that the pre-press proof should have the same appearance as the press print, that is to say in addition to matching the colors and dot gain of the press print, the pre-press proof should be on the same paper as the press print, the image gloss should be identical to that of the image produced on the press with printing inks and the paper surface in the image-free areas should remain unaffected.
Color image deposits formed on paper by the previously described electrophotographic process differ from color image deposits formed by printing inks in that the latter contains very much more binder material of the resinous or varnish type than the deposits formed by liquid toners in electrophotography; typically printing inks contain about 73% by weight of binder material, whereas prior art liquid toners contain only about 33% or less binder material by weight. Such relatively large quantity of varnish or resin in the printing ink deposit firstly fuses the image deposit to the paper and secondly transparentizes the color ink layers, which is necessary for the underlying colors to become visible and thereby to give the desired color combination effect.
Thus the previously referred to step of glazing the pre-press proof prepared by the electrophotographic process is essential firstly to fuse the image deposits to the paper and secondly to saturate the image deposits with resin to transparentize them to the extent where their appearance with regards color combination effect and gloss are the same as that of printing ink deposits.
Prior art glazing processes have three major disadvantages. Firstly, the application of a resin coating normally alters considerably the surface appearance of the paper sheet by imparting additional gloss thereto and/or even rendering transparent the paper itself particularly in those instances where it comprises publication type stock such as newsprint. Secondly, the resin layer increases the apparent optical density of color toner deposits, the extent of such increase being different for different colors, and also varying in accordance with the uniformity of the overcoated resin layer. Thirdly, as we have now found, the resin coating in print-free areas of the paper surface very considerably increases dot gain.
The term "dot gain" is well known in the graphic arts. For a complete description of this phenomenon, reference should be made for instance to the article "The Effect of the Spread-Function of Paper on Half-Tone Reproduction," by J. A. C. Yule et al., TAPPI/July 1967, Vol. 50, No. 7. For better understanding of this invention a brief description of the dot gain phenomenon will now be given relative to offset printing employing half-tone imagery, as is well known in the art.
A given percentage dot area of the film transparency employed to produce a printing plate can be exactly reproduced on the printed sheet provided the settings of the press, blanket pressure, ink/water balance, etc. are correct. It is found however, that even in such instances where no microscopically measurable dot distortion or spread occurs on the press, the density of the printed dot area is higher than would be expected by calculating the percentage dot area on the basis of the solid ink density on the printed sheet, that is to say, the measured density of the printed dot area of a given percentage is always equal to the density of a higher percentage dot area calculated on the basis of solid ink density. Such apparent increase in percentage dot over a given percentage dot in a known area is called dot gain.
It has been found that the extent of such dot gain depends mainly on the optical properties of the paper. The dots produce a shadow in the paper, which, the deeper it is in the paper, the more diffused and blurred it becomes. The more incident light between the dots is permitted by the optical nature of the paper to penetrate into the paper and scatter sideways towards and under the dot shadows, and the greater the depth of light penetration and hence higher incidence of diffused and blurred shadows, the more of the incident light will be absorbed and consequently less of the incident light will re-emerge from the dot-free areas and be reflected therefrom. The thus caused absorption of incident light results in a higher than expected density measurement of a given percentage dot area. Highly scattering paper surfaces, where the incident light is immediately reflected off the surface, produce therefore less dot gain than papers having a relatively transparent surface to incident light, such as uncoated papers with exposed fibers or highly reflective or glossy papers.
The half-tone screen ruling also affects dot gain to some extent because the above described effect of dot shadows on the incident light is amplified as the number of dots per unit area increases: thus, finer screen rulings produce higher dot gain than coarser screen rulings.
In 50% dot area a dot gain of 12 to 15% caused by the above factors on coated good quality paper is the accepted standard in normal printing. Higher dot gain indicates poor printing conditions and is generally unacceptable because it causes distortion of half-tone balance in the print.
It will be thus realized that the above referred to prior art methods of glazing electrophotographic pre-press proofs resulting in changes to the surface appearance of the paper, density increase of color deposits depending on glaze uniformity and increase in dot gain introduce very significant differences between such pre-press proofs and the printed paper, which limits considerably the usefulness of such proofs for the purposes of transparency correction approval.